Process for Increased Selectivity and Capacity for Hydrogen Sulfide Capture from Acid Gases

ABSTRACT

A process for selectively separating H 2 S from a gas mixture which also comprises CO 2  is disclosed. A stream of the gas mixture is contacted with an absorbent solution comprising one or more amines, alkanolamines, hindered alkanolamines, capped alkanolamines, or mixtures thereof. The H 2 S/CO 2  selectivity of the absorbent solution is preferably greater than about 4.0 for an acid gas loading [mol(CO 2 +H 2 S)/mol(amine)] between about 0.2 and about 0.6, and is achieved by reducing pH of the absorbent solution.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 14,980,634 filed on Dec. 28, 2015, which is incorporated byreference herein in its entirety.

BACKGROUND

The present invention relates to a process for removing acid gases fromnatural gas and other gas streams. In particular, it relates to aprocess for increasing the selectivity and capacity for hydrogen sulfideremoval from a natural gas stream using aqueous amine absorbents.

A number of different technologies are available for removing acid gasessuch as carbon dioxide, hydrogen sulfide, carbonyl sulfide. Theseprocesses include, for example, chemical absorption(amine/alkanolamine), physical absorption (solubility, e.g., organicsolvent, ionic liquid), cryogenic distillation (Ryan Holmes process),and membrane system separation. Of these, amine separation is a highlydeveloped technology with a number of competing processes in hand usingvariousamine/alkanolamine sorbents such as monoethanolamine (MEA),diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine(MDEA), diisopropylamine (DIPA), diglycolamine (DGA),2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ). Of these, MEA,DEA, and MDEA are the ones most commonly used. The acid gas scrubbingprocess using an amine in the purification process usually involvescontacting the gas mixture countercurrently with an aqueous solution ofthe amine in an absorber tower. The liquid amine stream is thenregenerated by desorption of the absorbed gases in a separate tower withthe regenerated amine and the desorbed gases leaving the tower asseparate streams. The various gas purification processes which areavailable are described, for example, in Gas Purification, Fifth Ed.,Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13:978-0-88415-220-0.

It is often necessary or desirable to treat acid gas mixtures containingboth CO₂ and H₂S so as to remove the H₂S selectively from the mixturewhile minimizing removal of the CO₂. While removal of CO₂ may benecessary to avoid corrosion problems and provide the required heatingvalue to the consumer, selective H₂S removal may be necessary ordesirable. Natural gas pipeline specifications, for example, set morestringent limits on the H₂S level than on the CO₂ since the H₂S is moretoxic and corrosive than CO₂: common carrier natural gas pipelinespecifications typically limit the H₂S content to 4 ppmv with a morelenient limitation on the CO₂ at 2 vol %. Selective removal of the H₂Smay enable a more economical treatment plant to be used and selectiveH₂S removal is often desirable to enrich the H₂S level in the feed to asulfur recovery unit.

The reaction kinetics with hindered amine sorbents allow H₂S to reactmore rapidly with the amine groups of the sorbent to form a hydrosulfidesalt in aqueous solution, but under conditions of extended gas-liquidcontact where equilibrium of the absorbed sulfidic species with CO2 isapproached, carbon dioxide can displace hydrogen sulfide from thepreviously absorbed hydrosulfide salt since carbon dioxide is a slightlystronger acid in aqueous solution than hydrogen sulfide (ionizationconstant for the first ionization step to H⁺ and HCO₃ ⁻ is approximately4×10⁻⁷ at 25° C. compared to 1×10⁻⁷ for the corresponding hydrogensulfide ionization) so that under near equilibrium conditions, selectiveH₂S removal becomes problematical, presenting a risk of excessive H₂Slevels in the effluent product gas stream.

An improvement in the basic amine process involves the use of stericallyhindered amines. U.S. Pat. No. 4,112,052, for example, describes the useof hindered amines for nearly complete removal of acid gases includingCO₂ and H₂S. U.S. Pat. Nos. 4,405,581; 4,405,583; 4,405,585 and4,471,138 disclose the use of severely sterically hindered aminecompounds for the selective removal of H₂S in the presence of CO₂.Compared to aqueous MDEA, severely sterically hindered amines lead tomuch higher selectivity at high H₂S loadings. Amines described in thesepatents include BTEE (bis(tertiary-butylamino)-ethoxy-ethane synthesizedfrom tertiary-butylamine and bis-(2-chloroethoxy)-ethane as well asEEETB (ethoxyethoxyethanol-tertiary-butylamine) synthesized fromtertiary-butylamine and chloroethoxyethoxyethanol). U.S. Pat. No.4,894,178 indicates that a mixture of BTEE and EEETB is particularlyeffective for the selective separation of H₂S from CO₂. U.S. Pat. No.8,486,183 describes the preparation of alkoxy-substituted etheramines asselective sorbents for separating H₂S from CO₂.

The use of hydroxyl-substituted amines (alkanolamines) such as thosementioned above has become common since the presence of the hydroxylgroups tends to improve the solubility of the absorbent/acid gasreaction products in the aqueous solvent systems widely used, sofacilitating circulation of the solvent through the conventionalabsorber tower/regeneration tower unit. This preference may, however,present its own problems in certain circumstances. A current businessdriver is to reduce the cost to regenerate and to recompress acid gasesprior to sequestration. For natural gas systems, the separation of theacid gases can occur at pressures of about 4,800-15,000 kPaa (about700-2,200 psia), more typically from about 7,250-8,250 kPaa (about1050-1200 psia). While the alkanolamines will effectively remove acidgases at these pressures, the selectivity for H2S removal can beexpected to decrease markedly both by direct physisorption of the CO₂ inthe liquid solvent and by reaction with the hydroxyl groups on the aminecompound. Although the CO₂ reacts preferentially with the aminonitrogen, higher pressures force reaction with the oxygens and under thehigher pressures, the bicarbonate/hemicarbonate/carbonate reactionproduct(s) formed by the reaction at the hydroxyl site is stabilizedwith a progressive loss in H₂S selectivity with increasing pressure.This effect can be perceived, for example, with MDEA(N-methyldiethanolamine). For example, 5M MDEA in aqueous solution doesnot absorb carbon dioxide under ambient conditions, but will form ahydrosulfide salt at the nitrogen. However, H₂S/CO₂ selectivitysignificantly reduces at high CO₂ pressure presumably due toO-carbonation of hydroxyl groups:

A similar trend is observed with the secondary aminoether,ethoxyethoxyethanol-t-butylamine (EEETB): at low pressures, thisabsorbent offers H₂S selectivity over CO₂ based on a faster reactionwith the hindered secondary amine group although a significant amount ofCO₂ can be absorbed by the hydroxyl group which has low affinity to H₂S.At higher pressures, however, the reaction yield of O-carbonationincreases, suppressing the H₂S/CO₂ selectivity achieved by the hinderedsecondary amine:

U.S. 2015/0027055 describes an absorbent system that can selectivelyabsorb H₂S from gas mixtures that also contain CO₂ and that can beregenerated at high pressure (greater than 10 bara) while maintainingvery low CO₂ solubility. The absorbent may include capped alkanolamines,i.e., alkanolamines in which one or more of the hydroxyl groups havebeen capped or converted into ether groups, includingN-(2-methoxyethyl)-N-methyl-ethanolamine (MDEA-(OMe),Bis-(2-methoxyethyl)-N-methylamine (MDEA-(OMe)2), 2-amino-prop-1-ylmethyl ether (AP-OMe), 2-methyl-2-amino-prop-1-yl methyl ether(AMP-OMe), 2-N-methylamino-prop-1-yl methyl ether (MAP-OMe),2-N-methylamino-2-methyl-prop-1-yl methyl ether (MAMP-OMe),2-N-ethylamino-2-dimethyl-prop-1-yl methyl ether, (EAMP-OMe),2-(N,N-dimethylamino)-ethyl methyl ether (DMAE-OMe), andMethoxyethoxyethoxyethanol-t-butylamine (M3ETB). The absorbent may alsoinclude more basic sterically hindered secondary and tertiary amines,including guanidines, amidines, biguanides, piperidines, piperazines,and the like, such as tetramethyguanidine, pentamethylguanidine,1,4-dimethylpiperazine, 1-methylpiperidine, 2-methylpiperidine,2,6-dimethylpiperidine, their hydroxyalkyl, e.g., hydroxyethylderivatives, and mixtures thereof.

In spite these advancements in absorbents, there still remains a needfor an absorption system that maintains a high selectivity andabsorption capacity for H₂S over a wide range of loadings.

SUMMARY OF THE INVENTION

We have now found that it is possible to achieve improved selectivityfor the removal of H₂S from gas mixtures also containing CO₂ bycontrolling the pH of the absorbent system. According to the presentinvention, a method for reducing the pH of an amine/alkanolamineabsorbent system is utilized to favor bicarbonate versus carbonateand/or hydrosulfide formation to maximize the stoichiometry ofabsorption. In a first aspect of the present invention, theconcentration of the amine/alkanolamine absorbent is reduced, resultingin the system having a lower pH. In a second aspect of the presentinvention, the pH of the amine/alkanolamine absorbent system is reducedby adding an acid to the system. The lower pH favors bicarbonateformation, increasing acid gas (H₂S and CO₂) loading, and increasing theselectivity of H₂S over CO₂ over a broad loading range. Particularamines that are found useful in the present invention are amines andalkanolamines, preferably sterically hindered amines and alkanolamines,and most preferably capped, sterically hindered amines such asmethoxyethoxyethoxyethanol-t-butylamine (M3ETB).

According to the present invention, a process for selectively separatingH₂S from a gas mixture which also comprises CO2, the process comprisingcontacting a stream of the gas mixture with an absorbent solutioncomprising one or more amines, wherein the H₂S/CO₂ selectivity of theabsorbent solution is greater than about 4.0, and preferably above about5.0, for an acid gas loading [mol(CO₂+H₂S)/mol(amine)] between about 0.2and about 0.6. Such a process also demonstrates superior H2S selectivityat relatively low acid gas loadings (up to 0.3[mol(CO₂+H₂S)/mol(amine)]).

Additional advantages of the present invention include reduced chemicalcosts (directly related to reduced amine usage), reduced viscosityresulting in reduced circulation energy, reduced corrosivity of theabsorption system, and reduced energy required to regenerate the reducedvolume of amine.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plot of the selectivity of H₂S over CO₂ as a function ofacid gas loading with different concentrations of M3ETB aqueoussolutions.

FIG. 2 is a plot of the CO₂ uptake as a function of treatment time byaqueous solutions of M3ETB.

FIG. 3 is a plot of the H₂S uptake as a function of treatment time byaqueous solutions of M3ETB.

FIG. 4 is a plot of the selectivity of H₂S over CO₂ as a function oftotal acid gas loading with different amine solutions as identified inTable 1.

FIG. 5 is a plot of the selectivity of H₂S over CO₂ as a function oftotal acid gas loading per volume of amine solution.

FIG. 6 is a plot of the CO₂ and H₂S uptake as a function of treatmenttime for the 36 wt % M3ETB solutions with and without phosphoric acid.

FIG. 7 is a plot of the selectivity of H₂S over CO₂ as a function oftotal loading with M3ETB aqueous solutions with and without phosphoricacid.

FIG. 8 is a plot of the CO₂ and H₂S uptake as a function of treatmenttime for the 36 wt % MDEA solutions with and without phosphoric acid.

FIG. 9 is a plot of the selectivity of H₂S over CO₂ as a function oftotal loading with MDEA aqueous solutions with and without phosphoricacid.

DETAILED DESCRIPTION

Conventional wisdom within the art of acid gas treating is that tertiaryamines in general form bicarbonates preferentially in reaction with CO₂in aqueous solution. Although hindered secondary amines are also able toform a carbamate with CO₂, the bicarbonate route becomes the preferredroute as steric hindrance increases. For absorption systems usingtertiary and sterically hindered amines, it is well known that there isa rapid equilibrium between bicarbonate and carbonate formation as shownbelow:

Stoichiometry Rate H₂S (Acid Base) H₂S + Amine ⇄ AmineH⁺ + HS⁻ 1.0 Veryfast CO₂ (Acid Base)

0.5 1.0 Slow

Because of their 1:1 stoichiometry with the amine group, the bicarbonateroute is more desirable because it favors enhanced selectivity for theabsorption of CO₂ and also for H₂S as the mercaptide salt in acid gasremoval processes. However, at the high pH of ˜12 of the initial loadingof the amine in aqueous solution carbonate formation is favored, butthat ties up two amine molecules for acid gas capture, providing a lessfavorable 0.5:1 stoichiometry. Furthermore, at higher amineconcentrations, there is less water available facilitating higher pH.

Described herein is a method for controlling/reducing the pH of ahindered amine/alkanolamine absorbent system to favor bicarbonate versuscarbonate and/or hydrosulfide formation to maximize the stoichiometry ofabsorption.

In a first aspect of the present invention, the pH of the hinderedamine/alkanolamine absorbent system is reduced. As demonstrated hereinbelow, this reduced pH favors bicarbonate formation, increases acid gas(H₂S and CO₂) loading and increases the selectivity of H₂S over CO₂ overa broad loading range.

In a second aspect of the present invention, the concentration of thehindered amine/alkanolamine absorbent is reduced, resulting in a dilutedamine solution necessarily having a lower pH. As demonstrated hereinbelow, this reduced pH favors bicarbonate formation, increases acid gas(H₂S and CO₂) loading, and increases the selectivity of H₂S over CO₂over a broad loading range.

The effect of reducing the concentration of the hinderedamine/alkanolamine absorbent is unexpected and counterintuitive.Traditionally, one strives to increase the absorbent concentration inorder to increase absorption. As demonstrated herein below, decreasingthe weight percentage of hindered amine/alkanolamine absorbent insolution effectively reduces the amount of carbonate that can be formedand thereby increases the amount of free amine available for acid gasrecovery as bicarbonate and hydrosulfide. Because absorption of HS iskinetically faster the selectivity for HS⁻ formation is favored overHCO₃ ⁻ formation. Additional advantages of the present invention includereduced chemical costs (directly related to reduced hinderedamine/alkanolamine usage), reduced viscosity resulting in reducedcirculation energy, reduced corrosivity of the absorption system, andreduced energy required to regenerate the higher loaded (1:1stoichiometry), but reduced volume of hindered amine/alkanolamine.

In order to provide a better understanding of the foregoing, thefollowing non-limiting examples are offered. Although the examples maybe directed to specific embodiments, they are not to be viewed aslimiting the invention in any specific respect.

The following experimental and analytical methods were used in theexamples. The process absorption unit (PAU) is a semi-batch system,comprising a water saturator, a stirred autoclave to which gas can befed in up-flow mode, and a condenser. The autoclave is equipped with apressure gauge and a type J thermocouple. A safety rupture disc isattached to the autoclave head. A high wattage ceramic fiber heater isused to supply heat to the autoclave. The gas flows are regulated byBrooks mass flow controllers and the temperature of the condenser ismaintained by a chiller. The maximum PAU working pressure andtemperature are 1000 psi (69 bar) and 350° C., respectively.

During runs at atmospheric pressure, the pH of the solution is monitoredin situ by using a Cole-Parmer pH probe installed in the bottom of theautoclave. This pH probe is limited by a maximum temperature andpressure of 135° C. and 100 psi, respectively. Therefore, before doingexperiments at pressure, the pH probe is removed and the autoclave iscapped. In both atmospheric and pressure runs, liquid samples arecollected directly to a vial (atmospheric runs) or to a stainless steelcylinder (pressure runs).

A custom LabVIEW program is used to control the PAU operation and toacquire experimental data (temperature, pressure, stirrer speed, pH, gasflow rate, and off-gas concentration).

The experiments described herein below were performed by flowing thetest acid gas mixture through the autoclave in which the amine solutionwas previously loaded. The acid gas mixture was fed to the bottom of thereactor by-passing the water saturator. The gases leaving the autoclavewere transferred through the condenser (maintained at 10° C.) in orderto remove any entrained liquids. A slip-stream of the off-gas leavingthe condenser was piped to a micron-GC (Inficon) for analysis while themain gas flow passed through a scrubber. After reaching breakthrough,nitrogen was used to purge the system.

The off-gas composition was measured using a custom-built micro GC. Themicro GC is configured as a refinery Gas Analyzer and includes fourcolumns (Mole Sieve, PLOT U, OV-1, PLOT Q) and four TCD detectors. Aslip stream of the off-gas was injected into the micro GC approximatelyevery 2 minutes. A small internal vacuum pump was used to transfer thesample into the micro GC. The nominal pump rate was ˜20 mL/min in orderto achieve 10× the volume of line flushes between the sample tee and themicro GC. The actual gas injected into the micro GC was ˜1 μL. The PLOTU column was used to separate and identify H₂S and CO₂, and the microTCD was used to quantify H₂S and CO₂.

Table 1 below identifies each amine solution prepared in water (MW=18g/mol; density =1 g/cm³) for Example 1:

TABLE 1 Molarity H₂O:amine MW Density (mol_(amine)/ Conc_(amine)Conc_(Water) Wt. % (mole Amine (g/mol) (g/cm³) L) (g_(amine)/L)(g_(Water)/L) Amine ratio) MDEA 119.2 1.04 2.17 258.66 736.20 26 18.8MDEA 119.2 1.04 3.05 363.56 646.33 36 11.8 EETB 161.2 0.94 2.17 349.80621.87 36 15.9 M3ETB 219.3 0.92 2.17 475.88 485.49 49.5 12.4 M3ETB 219.30.92 1.59 348.69 619.89 36 21.7 M3ETB 219.3 0.92 1.34 293.86 685.68 3028.3 M3ETB 219.3 0.92 0.90 197.37 789.48 20 48.5

Example 1 Selectivity Study

Three different amines were used to prepare amine absorbent solutionsfor Example 1. N-methyldiethanolamine (MDEA) is a commerciallyavailable, conventional amine treating absorbent having the followingstructure:

Ethoxyethanol-t-butylamine (EETB) is a commercially available, highlysterically hindered amine treating absorbent having the followingstructure:

Methoxyethoxyethoxyethanol-t-butylamine (M3ETB) is a methyl capped,sterically hindered amine:

Test conditions for Example 1 were as follows: gas feed composition: 10mol % CO2, 1 mol % H₂S, balance N₂; gas flow rate: 154 sccm;temperature: 40.8 ° C., pressure: 1 bar; volume: 15 mL; stirring rate:200 rpm.

FIG. 1 is a plot of the selectivity of H₂S over CO2 as a function ofacid gas loading with different concentrations of M3ETB aqueoussolutions. FIG. 2 is a plot of the CO₂ uptake as a function of treatmenttime by aqueous solutions of M3ETB. FIG. 3 is a plot of the H₂S uptakeas a function of treatment time by aqueous solutions of M3ETB.

The following conclusions are readily apparent from the plotted data ofFIGS. 1-3. The initial lower selectivity (up to 0.2 mol/mol amine) ofthe 30 wt % M3ETB solution in FIG. 1 is due to higher CO₂ and H₂S pickup(FIGS. 2 and 3, respectively) when compared to the 49.5 and 35.8 wt %M3ETB solutions. However, the higher selectivity (above 0.2 mol/molamine) of the 30 wt % solution is primarily due to the significantlyhigher H2S pickup when compared to the 49.5 and 35.8 wt % M3ETBsolutions. This increase in H₂S pickup (FIG. 3) is directly related toincreased initial bicarbonate formation versus carbonate formation,resulting in a higher amount of free amine for H₂S capture. Importantly,FIG. 1 further demonstrates that the 30 wt % M3ETB solution yields anoverall higher selectivity of H₂S over CO₂ for the commerciallydesirable acid gas loading range of 0.2 to 0.6, when compared to the49.5 and 35.8 wt % M3ETB solutions.

FIG. 4 is a plot of the selectivity of H₂S over CO₂ as a function oftotal acid gas loading with different amine solutions as identified inTable 1. These selectivity curves show that MDEA selectivity towards H₂Sis in general lower than obtained from the more highly stericallyhindered secondary amines EETB and M3ETB. This is expected since MDEA isa tertiary and less basic amine. As shown in FIG. 5, the observed MDEAacid gas loadings are also significantly lower than the loadings of themore sterically hindered amines EETB and M3ETB.

Furthermore, FIG. 4 allows one to compare the H₂S selectivity of variousM3ETB concentrations in water against EETB at 36 wt % concentration. The49.5 wt % M3ETB solution provides higher H₂S selectivity for lower acidgas loadings (up to ˜0.3 mol/mol amine) when compared with the EETB 36wt % solution. The 36 wt % M3ETB solution provides an even higher H₂Sselectivity over a broader range of acid gas loadings (up to ˜0.55mol/mol amine), and provides the highest H₂S selectivity for lower acidgas loadings (up to ˜0.3 mol/mol amine). The 30 wt % M3ETB solutionprovides even higher selectivity up to higher acid gas loadings (up to˜0.65 mol/mol amine). As can be readily understood by a person havingordinary skill in the art, each of these diluted M3ETB solutionsprovides an improved H₂S selectivity up to higher acid gas loadings, andcan be predictably tuned to meet specific acid gas removal applications.

FIG. 5 is a plot of the selectivity of H₂S over CO2 as a function oftotal acid gas loading per volume of amine solution. The M3ETB solutionshave a higher capability for removing H₂S than MDEA and EETB up to ahigher acid gas loading per volume (˜0.8 mol/L solution). As can bereadily understood by a person having ordinary skill in the art, thisrepresents potential energy savings from a reduced circulation rate whenusing diluted M3ETB solutions.

Example 2 M3ETB pH Study

Two amine solutions were prepared using M3ETB, a first solution with 36wt % M3ETB, and a second solution of 36 wt % M3ETB with 0.5 wt % H₃PO₄,both in water. Test conditions for Example 2 were as follows: gas feedcomposition: 10 mol % CO₂, 1 mol % H₂S, balance N₂; gas flow rate: 154sccm; temperature: 40.8° C., pressure: 1 bar; volume: 15 mL; stirringrate: 200 rpm.

The effect of phosphoric acid on M3ETB acid gas absorption behavior canbe observed in FIGS. 6 and 7. FIG. 6 is a plot of the CO₂ and H₂S uptakeas a function of treatment time for the 36 wt % M3ETB solutions with andwithout phosphoric acid. FIG. 7 is a plot of the selectivity of H₂S overCO₂ as a function of total loading with M3ETB aqueous solutions with andwithout phosphoric acid. The presence of phosphoric acid increases theloading of both CO₂ and H₂S gases, thereby impacting the selectivity.This result is in line with the prior conclusion from Example 1 thatbicarbonate formation is favored with lower pH, thereby increasing thefree amine concentration in solution available for rapid H₂S capture.

Example 3 MDEA pH Study (Comparative)

Two amine solutions were prepared using MDEA, a first solution with 36wt % MDEA, and a second solution of 36 wt % MDEA with 0.5 wt % H₃PO₄,both in water. Test conditions for Example 2 were as follows: gas feedcomposition: 10 mol % CO₂, 1 mol % H2S, balance N₂; gas flow rate: 154sccm; temperature: 40.8° C., pressure: 1 bar; volume: 15 mL; stirringrate: 200 rpm.

The effect of phosphoric acid on MDEA acid gas absorption behavior canbe observed in FIGS. 8 and 9. FIG. 8 is a plot of the CO₂ and H₂S uptakeas a function of treatment time for the 36 wt % MDEA solutions with andwithout phosphoric acid. FIG. 9 is a plot of the selectivity of H₂S overCO₂ as a function of total loading with MDEA aqueous solutions with andwithout phosphoric acid. There is no significant difference observedbecause it is known that bicarbonate is preferentially formed duringCO₂-MDEA reactions. Nevertheless, it is observed that the acidified MDEAsolution may lead to slightly higher bicarbonate formation. FIG. 9 inparticular demonstrates a reduced H₂S selectivity for the MDEA solutionwith phosphoric acid, which is the opposite effect as the highlysterically hindered M3ETB.

ADDITIONAL EMBODIMENTS

According to certain teachings of the present invention, a process forselectively separating H₂S from a gas mixture which also comprises CO2is disclosed, the process comprising contacting a stream of the gasmixture with an absorbent solution comprising one or more amines. TheH₂S/CO₂ selectivity of the absorbent solution is greater than about 4.0,and preferably greater than 5.0, for an acid gas loading[mol(CO₂+H₂S)/mol(amine)] between about 0.2 and about 0.6. The one ormore amines is selected from the group consisting of amines,alkanolamines, sterically hindered akanolamines, and mixtures thereof,and is preferably methoxyethoxyethoxyethanol-t-butylamine (M3ETB),ethoxyethanol-t-butylamine (EETB), or N-methyldiethanolamine (MDEA). Thesterically hindered alkanolamine is preferably a capped amine.

Another embodiment of the present invention is a method for increasingthe selectivity of an absorption process for H₂S absorption from a gasmixture which also comprises CO₂, the absorption process having anabsorbent solution comprising one or more amines, the method comprisingreducing the pH of the absorbent solution. This pH reducing step isaccomplished by diluting the absorbent solution, or by adding an acid tothe absorbent solution, the acid being selected from phosphoric acid andsulfuric acid. The one or more amines is selected from the groupconsisting of amines, alkanolamines, sterically hindered akanolamines,and mixtures thereof, and is preferably M2ETB, EETB, or MDEA.

Yet another embodiment of the present invention is a process forselectively separating H2S from a gas mixture which also comprises CO₂,the process comprising contacting a stream of the gas mixture with anabsorbent solution comprising M3ETB. The M3ETB concentration in theabsorbent solution is less than about 36 wt %, and preferably between 25and 30 wt %. The H₂S/CO₂ selectivity of the absorbent solution isgreater than about 4.0, and preferably greater than 5.0, for an acid gasloading [mol(CO₂+H₂S)/mol(amine)] range between about 0.2 and about 0.6.

Still another embodiment of the present invention is a system forselectively absorbing H2S from a raw gas stream which also comprisesCO₂, the system comprising an absorber tower for contacting the raw gasstream countercurrently with an aqueous amine stream to create a spentamine stream comprising at least a portion of the H₂S from the raw gasstream, and a regeneration tower for creating a regenerated amine streamand a desorbed acid gas stream. The H₂S/CO₂ selectivity of the aqueousamine stream is greater than about 4.0, preferably greater than about5.0, for an acid gas loading [mol(CO₂+H₂S)/mol(amine)] between about 0.2and about 0.6. The aqueous amine stream preferably comprisesmethoxyethoxyethoxyethanol-t-butylamine (M3ETB) in a concentration lessthan about 36 wt %, and preferably between 25 and 30 wt %. The H₂S/CO₂selectivity of the system is increased by reducing the pH of the aqueousamine stream, preferably by lowering the amine concentration in theaqueous amine stream.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings therein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andsprit of the present invention. Unless otherwise indicated, all numbersexpressing quantities of ingredients, properties, reaction conditions,and so forth, used in the specification and claims are to be understoodas approximations based on the desired properties sought to be obtainedby the present invention, and the error of measurement, etc., and shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Whenever anumerical range with a lower limit and an upper limit is disclosed, anumber falling within the range is specifically disclose. Moreover, theindefinite articles “a” or “an”, as use in the claims, are definedherein to mean one or more than one of the element that it introduces.

What is claimed is:
 1. A method for increasing the selectivity of anabsorption process for H₂S absorption from a gas mixture which alsocomprises CO₂, the absorption process having an absorbent solutioncomprising one or more amines, the method comprising reducing the pH ofthe absorbent solution.
 2. The method of claim 1, wherein the pHreducing step comprises diluting the absorbent solution.
 3. The methodof claim 2, wherein the diluted absorbent solution is less than 36 wt %.4. The method of claim 2, wherein the diluted absorbent solution is lessthan 30 wt %.
 5. The method of claim 1, wherein the pH reducing stepcomprises adding an acid to the absorbent solution.
 6. The method ofclaim 3, wherein the acid is selected from phosphoric acid and sulfuricacid.
 7. The method of claim 1, wherein the one or more amines isselected from the group consisting of amines, alkanolamines, stericallyhindered akanolamines, and mixtures thereof.
 8. The method of claim 7,wherein the sterically hindered alkanolamine is a capped alkanolamine.9. The method of claim 7, wherein the amine is M3ETB.
 10. The method ofclaim 7, wherein the amine is EETB.
 11. The method of claim 7, whereinthe amine is MDEA.
 12. The method of claim 1, wherein the H₂S/CO₂selectivity of the absorbent solution is greater than about 5.0 for anacid gas loading [mol(CO₂+H₂S)/mol(amine)] range between about 0.2 andabout 0.55.
 13. The method of claim 1, wherein the H₂S/CO₂ selectivityof the absorbent solution is greater than about 6.0 for an acid gasloading [mol(CO₂+H₂S)/mol(amine)] range between about 0.2 and about0.55.
 14. The method of claim 1, wherein the H₂S/CO₂ selectivity of theabsorbent solution is greater than about 7.0 for an acid gas loading[mol(CO₂+H₂S)/mol(amine)] range between about 0.2 and about 0.55.
 15. Aprocess for selectively separating H₂S from a gas mixture which alsocomprises CO₂, the process comprising: contacting a stream of the gasmixture with an absorbent solution comprising one or more amines,wherein the absorbent solution has a reduced pH.
 16. The process ofclaim 15, wherein the pH of the absorbent solution is reduced bydilution.
 17. The process of claim 16, wherein the diluted absorbentsolution is less than 36 wt %.
 18. The process of claim 16, wherein thediluted absorbent solution is less than 30 wt %.
 19. The process ofclaim 15, wherein the pH of the absorbent solution is reduced byaddition of an acid.
 20. The process of claim 19, wherein the acid isselected from phosphoric acid and sulfuric acid.
 21. The process ofclaim 15, wherein the H₂S/CO₂ selectivity of the absorbent solution isgreater than about 5.0 for an acid gas loading [mol(CO₂+H₂S)/mol(amine)]range between about 0.2 and about 0.55.
 22. The process of claim 15,wherein the H₂S/CO₂ selectivity of the absorbent solution is greaterthan about 6.0 for an acid gas loading [mol(CO₂+H₂S)/mol(amine)] rangebetween about 0.2 and about 0.55.
 23. The process of claim 15, whereinthe H₂S/CO₂ selectivity of the absorbent solution is greater than about7.0 for an acid gas loading [mol(CO₂+H₂S)/mol(amine)] range betweenabout 0.2 and about 0.55.